Asymmetrical digital subscriber line (ADSL) technology has evolved over the years of development from the first generation ADSL, the second generation ADSL2 and ADSL2+, to the latest very high speed digital subscriber line2 (VDSL2). With the increase of frequency band, bandwidth also increases. ADSL and ADSL2, with the downlink spectrum lower than 1.1 MHz, provide downlink rates as high as 8 Mbps. ADSL2+expands the downlink bandwidth to 2.2 MHz and provides a maximum downlink rate of 24 Mbps. VDSL2 uses a spectrum of up to 30 MHz on the downlink, and provides an uplink-downlink symmetrical rate of 100 Mbps.
As the frequency band in xDSL technology increases, however, crosstalk becomes a nuisance, especially in a high frequency band. There are near-end crosstalk (NEXT) and far-end crosstalk (FEXT), as shown in FIG. 1 and FIG. 2. NEXT does no significant harm to system performance while FEXT has a severe impact on line transmission.
For example, when xDSL services are activated for multiple subscribers in a bundle of cables, some lines may suffer from low transmission rate and instability or even xDSL services fail to be activated because of FEXT, leading to a low activation rate of the digital subscriber line access multiplexer (DSLAM).
Some operators have worked out their specifications on the management of spectrum applications to avoid crosstalk between devices in various locations. The technologies and methods currently used to address crosstalk issues include multi-subscriber detection technology, maximum likelihood multi-subscriber detection technology, and interference cancellation multi-subscriber detection technology.
The current technologies, however, are complicated and require a large amount of calculation. In addition, the technologies only optimize the performance of DSL lines or the system based on some statistic characteristics of crosstalk instead of providing quantitative crosstalk tests on multiple lines. As a result, the transmission performance of DSL lines cannot be optimized significantly.